Yes, even in an empty session dragging a new device - instrument, effect, VST - always takes few seconds in which Live's GUI freezes, although playback continues in the background.
Adding that same device(s) afterwards is instant, so it's perhaps a buffering of some sort?

Update: the issue started happening again shortly after (a few hours). I thought this was clearly a sign of the index database slowing things down, but my Ableton application on an SSD and I tried not loading most of my samples into the sidebar in Ableton so it wouldn't index, and it still happens. Not sure.